Hiding inside the identity of someone else — someone recently disappeared, or murdered, for example — is a theme running through all sorts of crime fiction (Patricia Highsmith's Ripley mysteries) as well as cinematic variations on that theme (Michelangelo Antonioni's "The Passenger"). In the French thriller "The Big Picture" now at the Music Box, Romain Duris plays Paul, a successful but bitter attorney whose boss (Catherine Deneuve) is ready to hand the firm over to his control. Once upon a time Paul dreamed a dream of becoming a photographer.

That dream is dead; he now lives in a Paris suburb with a bitter wife (Marina Fois), who has sacrificed her writing career to raise their two children.

She is having an affair with a free-lance photographer (Eric Ruf), marginally successful but charismatic. He is the man her husband didn't have the guts to become himself.

Then comes the change-up. Confronting his wife's lover, Paul accidentally kills the man in an argument. Thus begins Paul's transformation: Slipping out of his old life with improbable but entertaining ease, he assumes the photographer's identity, embarks on an extended clandestine working trip to Croatia, where the career he always wanted clicks miraculously into place.

Preposterous but engaging, the film wouldn't work at all if the central irony wasn't pretty rich. Paul wants only isolation and seclusion, but on the Croatian seaside he meets a magazine editor (Niels Arestrup, one of the great, grizzled character actors in movies) who passes his photo samples along to a lovely staffer (Branka Katic) who becomes interested in his career. Everybody wonders why this man Greg, formerly known as Paul, is so skittish about having his picture taken, or talking to the press. We watch the film, directed by Eric Lartigau from a script he worked on with several other adapters, and wonder what will greet Paul/Greg at the end of his road — absolution, a comeuppance, or something else?

The novel from which it's taken, by Douglas Kennedy, was set in Connecticut. Paris, Croatia and environs suit the reworked story. The narrative improbabilities are legion: Do most well-to-do Parisians honestly have hazmat suits lying around in case there's a bloody corpse to clean up after? Nervous like a ferret, Duris overdoes some of the twitchiness in the early going, but that's one of the picture's guiding notions. "The Big Picture," which ends on a dying fall rather than a thundering climax, is about a man who isn't comfortable in his own skin until he metaphorically slips on someone else's for size.